Scanning radiographic equipment differs from conventional radiography in that it employs a narrowly collimated beam of radiation, typically x-rays, formed into, for example, a fan beam, rather than a broad area cone beam. The compact beam size allows the replacement of an image forming sheet of radiographic film, used with conventional radiographic equipment, with a small area array of electronic detector elements. Further, the scanning allows the collection of data over a much broader area than would be practical with a single x-ray cone beam.
The electronic detector elements receiving the transmitted radiation produce electrical signals which may be converted to digital values by an analog-to-digital converter for the later development of an image or for other processing by computer equipment. The ability to quantify the measurement of the transmitted radiation, implicit in the digitization by the analog to digital converter, allows not only the formation of a radiographic “attenuation” image but also the mathematical analysis of the composition of the attenuating material by dual energy techniques. Such dual energy techniques quantitatively compare the attenuation of radiation at two energies to distinguish, for example, between bone and soft tissue. This makes possible the measurement of bone mass, such measurement being important in the treatment of osteoporosis and other bone diseases.
Bone densitometers, particularly central dual-energy absorptiometers (central DXA or DEXA), are used to measure the proximal femur for bone mineral content (BMC) and bone mineral area density (g/cm2) (commonly referred to as bone mineral density or BMD). The BMD of the proximal femur is used to diagnose osteoporosis and predict future hip and other osteoporotic fracture risk. When performing a central DXA scan to measure the BMD of the proximal hip, only a couple of centimeters below the lessor trochanter to a couple of centimeters above the head of the femur is typically imaged.
Reduction of fracture risk is commonly accomplished by anti-resorptive medications, one class of which is bisphonates (e.g., alendronate sodium, risedronate sodium, ibandronate sodium, zoledronic acid, etc.). It has been determined, that bisphosphonates, and likely other anti-resorptives used to reduce fracture risk in primary and secondary osteoporosis, are associated with atypical femoral fractures (AFFs). Atypical femoral fractures seem to be stress fractures that develop over a period of time, in some cases, months to years. AFFs occur below the lessor trochanter and above the supracondylar flare and are often bilateral. Developing AFFs are associated with focal thickening of the lateral cortex due to local endosteal and periosteal reactions. Additionally, as AFFs progress, they may be associated with a transverse radiolucent line. Developing AFFs can be seen with a number of different radiologic modalities, including central DXA, x-ray radiographs, magnetic resonance imaging (MRI), computed tomography (CT) scans, and bone scans. While developing AFFs may be seen on DXA, a typical clinical DXA exam encompasses only a small fraction of the region where AFFs occur. While the DXA region when performing proximal femur BMD measurements can be extended, typically only one half of the average adult femur can be imaged. It is also possible to detect signs that are associated with AFFs by using a separate, single-energy, high resolution scan capable of imaging the entire femur for visualization of traits associated with an incomplete or developing AFF.
Regarding the form of the radiation used in bone DXA systems, the compact beam of radiation used in scanning radiographic systems allows the use of limited area detectors permitting high resolution with relatively lower cost. Further, the images formed by a compact beam are potentially more accurate than those produced by a typical broad beam radiographic system. The accuracy arises from the limited divergence of the rays of the beam as compared to a broad area cone beam. This narrow collimation of the fan beam reduces “parallax” in the projected image, particularly of anatomical planar surfaces that are nearly parallel with the central ray of the beam.
The compact beam of radiation, however, also requires increased scanning motion if large areas are to be measured. In a fan beam system, typically the fan beam will be scanned in a raster or “zig-zag” pattern over the area to be measured, each line of the scan forming a scan image separated by somewhat less than the width of the fan beam to ensure complete illumination of the entire volume of the imaged object. The direction of scanning is generally perpendicular to the direction of the radiation and the plane of the fan beam. In general, each scan path is generally parallel to a longitudinal axis of a patient being scanned.